A light-emitting diode (LED) constant-current control circuit capable of lighting using pulse width modulation (PWM) includes, for a light source unit having multiple LEDs, a comparator amplifier (hereinafter, called an “op-amp”), a first negative channel metal oxide semiconductor (NMOS) transistor as a first switch, a second NMOS transistor as a second switch, and a shunt resistor. A gate of the first NMOS transistor is connected to a drain of the second NMOS transistor, and a drain of the first NMOS transistor is connected in series downstream (low side) of the light source unit (see e.g., Japanese Patent Document JP-A-2004-134147). The shunt resistor is connected in series to the light source unit and the first NMOS transistor.
The first NMOS transistor controls current supplied to the light source unit in response to a voltage provided to a gate terminal.
The shunt resistor detects a voltage depending on the current flowing to the light source unit. The shunt resistor detects the current (hereinafter, called “LED current”) flowing to the light source unit as the voltage (hereinafter, called “detection voltage”) between both ends of the shunt resistor.
The detection voltage is applied to an inverting input terminal (negative input terminal) of the op-amp, and a reference voltage is applied to a non-inverting input terminal (positive input terminal) of the op-amp. The op-amp transmits a comparison output signal to the gate of the first NMOS transistor. When the detection voltage detected by the shunt resistor is applied to the negative input terminal of the op-amp, the op-amp compares the detection voltage with the reference voltage applied to the non-inverting input terminal (positive input terminal) and applies a voltage (comparison output signal) according to the comparison result to the gate of the first NMOS transistor, thereby controlling ON/OFF operations of the first NMOS transistor.
The second NMOS transistor receives a PWM signal at a high level (H) or a low level (L) to provide a control signal at the low level (L) or the high level (H).
When the PWM signal is turned off, the second NMOS transistor receives the PWM signal at the high level (H) to be in an ON state. An output of the op-amp is maintained at the high-level, and the first NMOS transistor is in an OFF state, so that LED current does not flow.
When the PWM signal is turned on, the second NMOS transistor receives the PWM signal at the low-level (L) to be in the OFF state, and the first NMOS transistor is in the ON state, so that current flows to the LED. The output of the op-amp is a comparison output signal based on comparing the detection voltage detected by the shunt resistor with the reference voltage applied to the positive input terminal of the op-amp.
As described above, as constant-current control is performed in response to ON/OFF of the PWM signal, the LED current is maintained (lighting control of the LED) at a given level, thereby lighting up the LED with a proper intensity.
In the foregoing circuit, after the PWM signal is turned on, the comparison output signal of the op-amp rapidly drops, and a higher LED current tends to flow. Accordingly, until the level of the LED current becomes stable at a desired level depending on the comparison output signal of the op-amp after a DC power supply is turned on, the LED current rapidly increases, and an overshoot occurs.
In addition, since the output of the op-amp rapidly changes, the op-amp easily oscillates. In the case where a phase compensation is increased using a condenser to prevent oscillation of the op-amp, the period of overshoot of the LED current increases, feedback control does not follow the PWM signal, and thus response characteristics of the op-amp may be deteriorated. Particularly, when the on-duty of the PWM signal is shortened, ON/OFF control cannot be performed, and thus constant-current control cannot be performed.
In addition, since the cathode side of the LED performs the constant-current control, a ground fault of the cathode of the LED has to be considered, and this tends to increase the complexity of the circuit configuration.
In addition, when a ground fault of the anode side of the LED occurs, a high current flows to an LED driving unit for supplying the LED current to the LED, and thus this causes a breakdown in the LED driving unit.